1. Technical Field
The present invention is related to a surface emitting type semiconductor light emitting element equipped with a distributed Bragg reflector and an active layer which are sequentially grown on a semiconductor substrate.
2. Background Art
There are semiconductor light emitting elements, such as light emitting diodes (LED's) and semiconductor lasers (LD's: Laser Diodes) which are configured with DBR's (Distributed Bragg Reflectors) in order to improve the light emitting efficiency thereof (Japanese Unexamined Patent Publication No. 6(1994)-196681, Japanese Unexamined Patent Publication No. 2009-070929, etc.). In a semiconductor light emitting element of the surface emitting type, DBR's provided as layers above and below a light emitting layer generally contribute to increases in light emitting efficiency by increasing the reflectance of a specific wavelength as structures that increase resonance. DBR's are constituted by multiple layered films, in which layers having a relatively high refractive index (high refractive index layers) and layers having a relatively low refractive index (low refractive index layers) are alternately laminated. The wavelength which is reflected by a DBR is extremely sensitive to the thicknesses of the films that constitute the DBR. Accordingly, the light emitting efficiency of a semiconductor light emitting element equipped with a DBR depends greatly on the change in emitted light intensity (light emitting efficiency) when the light emitting wavelength shifts. That is, the increase in light emitting efficiency obtained by a DBR and the degree of change in the light emitting efficiency due to shifts in wavelengths are in a trade off relationship.
Meanwhile, it is known that the light emitting spectra of semiconductor light emitting elements change greatly due to changes in ambient temperature, and that the light emission wavelengths shift toward the longer wavelength side as temperatures become higher. For example, a wavelength shift of approximately 10 nm toward the long wavelength side occurs in GaAs—AlAs LED's when the temperature of a utilization environment rises approximately 40° C. For this reason, there is a problem that the light emission intensity of semiconductor light emitting elements equipped with DBR's varies greatly due to changes in ambient temperature.
Japanese Unexamined Patent Publication Nos. 2009-070929 and 2003-332615 propose methods for solving this problem of variation in light emitting efficiency caused by changes in ambient temperature.
Japanese Unexamined Patent Publication No. 2009-070929 discloses an element configuration in which a plurality of multiple layer film reflecting layers that respectively reflect light having longer wavelengths and light having shorter wavelengths than the light spectrum generated by an active layer under the active layer, to effectively cause the light spectrum to become a wide band light spectrum, thereby reducing the influence of wavelength shift caused by changes in ambient temperature.
Japanese Unexamined Patent Publication No. 2003-332615 discloses an element configuration in which a notch filter constituted by a first multiple layer film layer, a second multiple layer film layer, and a spacer layer provided between these two layers is provided, to reduce the influence of wavelength shift caused by changes in ambient temperature.
Meanwhile, exposure devices, in which multiple semiconductor light emitting elements are arranged one dimensionally or two dimensionally, are employed as exposure heads for printers and scanners. Uniformity of light intensity among the arranged plurality of semiconductor light emitting elements is required in such arrayed exposure devices. Here, the expression “light intensity” refers to the intensity of light which is integrated across the light emitting wavelength band of the light emitting elements.
The light intensities emitted by individual semiconductor light emitting elements among a plurality of light emitting elements which are employed in an exposure device also change due to wavelength shifts caused by changes in ambient temperature. Further, a phenomenon that the manner in which the light intensity changes is different for each element has been observed. For this reason, there is a problem that uniformity of light intensities cannot be maintained among a plurality of semiconductor light emitting elements which are employed in an exposure device.
Japanese Unexamined Patent Publication No. 2010-219220 proposes a light emitting device that suppresses changes in light intensity of light emitting elements due to temperature changes, by providing a reflecting layer beneath an active layer, and a surface having recesses and protrusions at a plurality of distances from the reflecting layer above the active layer.